Management of key metabolic intersections is a critical issue in bacterial economy. One such intersection, the interconversion of a-ketoglutarate and glutamate, is particularly important and interesting because it represents the point of coupling between central carbon metabolism and central nitrogen metabolism. Synthesis of the enzymes of the tricarboxylic acid branch of the Krebs cycle (citrate synthase, aconitase and isocitrate dehydrogenase) that produce a-ketoglutarate and the enzymes of nitrogen assimilation (glutamine synthetase, glutamate synthase and glutamate dehydrogenase) are encoded by genes that are subject to multiple forms of regulation. This project seeks to understand in detail the molecular mechanisms that regulate these genes in Bacillus subtilis, a well studied model Gram-positive bacterium, and Listeria monocytogenes, a model intracellular pathogen whose metabolic regulation is relatively unexplored. The proposal focuses on three transcriptional regulators (CcpC, GltC and GlnR) and two enzymes (glutamate dehydrogenase and aconitase) that have post-transcriptional regulatory functions. The aims of this proposal are: (a) to determine the molecular mechanisms that control the synthesis and activity of the tricarboxylic acid branch enzymes;(b) to reveal the molecular mechanisms that control the synthesis of glutamate and glutamine;and, (c) to determine the role of aconitase as a post-transcriptional regulator of sporulation in B. subtilis and of iron metabolism and virulence genes in L. monocytogenes. These issues will be addressed using a combination of genetic and biochemical techniques. The phenotypes of mutants defective in each of the regulatory factors will be determined and the proteins will be purified and tested using in vitro transcription and DNA and RNA binding experiments. Since L. monocytogenes mutants that lack two repressors of the Krebs cycle genes have a significant defect in virulence in a mouse model, the molecular basis for this defect will be explored. PUBLIC HEALTH RELEVANCE: Maintaining the proper balance in intracellular metabolism is a critical factor in bacterial growth, survival and pathogenesis. This proposal aims to unravel the intricate mechanisms by which two related bacterial species, Bacillus subtilis (a non-pathogen) and Listeria monocytogenes (the causative agent of listeriosis), control the flow of metabolites between central carbon and central nitrogen pathways. The knowledge gained will provide a deeper understanding of pathogenic mechanisms and may aid in designing novel inhibitors of bacterial growth and pathogenesis.